Sift the flour into a large bowl. Add the butter, then use a pastry cutter or two butter knives to cut the butter into the flour until fully incorporated and the mixture resembles small peas.

In a small bowl, whisk together the egg yolk and sour cream, then add to the flour mixture. Mix well to form a dough. Shape the dough into a disc, then dust with flour. Divide the disk into thirds, then wrap each piece in wax paper. Refrigerate for several hours or overnight.

Meanwhile, to prepare the filling, in a food processor combine both sugars, the pecans and cinnamon. Pulse until finely ground, then set aside.

When ready to assemble, heat the oven to 375 degrees. Line a baking sheet with either a silicone baking mat or parchment paper.

Work with one piece of the dough at a time, leaving the others in the refrigerator until needed. On a lightly floured surface, roll out each piece of dough into a circle about 1/8 inch thick and 12 to 13 inches around. Sprinkle a bit of the filling over the dough, then use a rolling pin to gently press the topping into the dough. Use a paring knife or pizza cutter to cut the circle into 16 wedges (make 8 cuts across the circle).

One at a time, roll up each wedge starting with the wide end. Gently curl the ends inward to resemble a croissant. Sprinkle a bit of the filling over the rolled rugelach, then place on the prepared baking sheet. Repeat with the remaining wedges, then repeat the entire process with the remaining thirds of the dough.

Bake for 25 to 30 minutes, or until browned on top and the sugar has melted and caramelized around the sides of the cookies. Transfer to a wire rack to cool. Store in airtight containers.